A cathode material made of LiMnPO4 is a material that can be expected to have a higher battery reaction voltage and an energy density approximately 20% higher than a cathode material made of LiFePO4. Therefore, the cathode material made of LiMnPO4 is expected to be developed as a secondary battery for an electrical vehicle.
However, a lithium-ion rechargeable battery including a cathode which includes a cathode material made of LiMnPO4 has the following problems. (1) The low electron conductivity of bulk LiMnPO4, (2) the low Li diffusivity of the bulk LiMnPO4, and (3) the anisotropic and large volume change of a LiMnPO4 crystal attributed to a battery reaction which is caused by the Jahn-Teller effect of a manganese ion (Mn2+). Due to these problems, in a lithium-ion rechargeable battery, the activation energy for intercalating and deintercalating lithium ions into and from a cathode becomes high. As a result, in the lithium-ion rechargeable battery, battery characteristics at a low temperature significantly degrade.
In order to improve the battery characteristics of a lithium-ion rechargeable battery at a low temperature, active studies are underway regarding LiFexMn1-xPO4 (0<x<1) in which a portion of Mn in LiMnPO4 is substituted with Fe (for example, refer to Patent Document 1). In LiFexMn1-xPO4, Fe forms a solid solution, and thus electron conductivity among particles improves more than LiMnPO4. As a result, in a lithium-ion rechargeable battery including a cathode which includes a cathode material made of LiFexMn1-xPO4, charge and discharge performance improves.